Pyrolysis reactions produce a complex and variable reaction mixture including vaporous products which are liquid at room temperature. Pyrolysis is a general term for the thermal decomposition of any organic material (e.g., wood, plants, or fossil fuels) and can occur during a combustion process or in the absence of combustion. In the former, the oxidation of a portion of the organic material provides the heat required to vaporize and decompose the remainder. In the absence of combustion, heat is supplied indirectly from some other source (e.g., radiation, a solid or gaseous heat carrier or conduction through reactor walls).
Therefore, as used here and hereinafter, the term "pyrolysis" is defined as the thermal degradation of a material primarily into smaller, or simpler, molecules and into a minor portion of a residue, including ash and a residual polymeric material, or char. Similarly, here and hereinafter, the terms "pyrolysis product" and "pyrolysis liquid" are defined as the degradation products, or the smaller, simpler molecules, that result from pyrolysis, as opposed to the residue, including the pyrolytic char.
Therefore, pyrolysis of an organic material or biomass produces liquids (condensable vapors), gases (noncondensable vapors) and solids (char and ash), in varying proportions, depending upon reaction conditions. The term "pyrolysis product" is hereby further defined as the condensable vapors and noncondensable vapors resulting from pyrolysis. The term "pyrolysis liquid" is hereby further defined as the condensable vapors resulting from pyrolysis. The residue of char and ash is included neither in the definition of a pyrolysis product or a pyrolysis liquid. The pyrolysis liquid can be further subdivided into water-soluble condensable vapors and water-insoluble condensable vapors. The water-soluble condensable vapors are the desirable active ingredients for smoke flavoring.
Use of pyrolysis liquids, or solutions including pyrolysis liquids, as a replacement for smoking foodstuffs by direct contact with smoke produced from burning wood has become a standard industry practice. When applied to the surface of meats and other proteinaceous foodstuffs, conventional pyrolysis liquids (and solutions including pyrolysis liquids), termed "liquid smokes", not only give the foodstuff a characteristic smoke flavor, but react with the proteins to produce a coloring typical of smoked foodstuffs.
One such commercial liquid smoke product is the aqueous liquid smoke flavoring disclosed in Hollenbeck U.S. Pat. No. 3,106,473. This flavoring product is produced by slow pyrolysis or partial combustion of wood with limited access to air, followed by subsequent solvation of the desirable smoke constituents into water. The water-soluble condensable vapors are used for smoke flavor, while the water-insoluble condensable vapors which include tar, polymers, polycyclic aromatic hydrocarbons (including benzo(a)pyrene), waxes and other compounds unsuitable or undesirable for use in food applications are discarded.
Another method of producing a pyrolysis liquid for smoke flavoring foods is the fast pyrolysis of wood or cellulose disclosed in Underwood et al. in U.S. Pat. No. 4,876,108. The pyrolysis liquids produced by the fast pyrolysis process are collected and diluted with water to achieve a partial phase separation and to provide an aqueous liquid smoke-flavored solution.
Regardless of whether wood or cellulose is pyrolyzed by a slow pyrolysis method or by a fast pyrolysis method, the resulting smoke-flavored pyrolysis liquid, or a solution including the pyrolysis liquid, imparts a stronger than desirable smoke flavoring to some foodstuffs for a given degree of smoke coloring for the tastes of some consumers. Even though such consumers prefer a very mild to little smoke flavor, these consumers still prefer a flavored foodstuff, especially meat, having the typical full-brown color associated with well-smoked foodstuffs. Even though a need exists for such a mild, smoke-flavored pyrolysis liquid, or solution including such a pyrolysis liquid, apparently none is presently available.